1. Field of the Invention
The present invention relates generally to papermaking, and relates more specifically to a structured forming fabrics employed in papermaking. The invention also relates to a structured forming fabric having deep pockets.
2. Discussion of Background Information
In the conventional fourdrinier papermaking process, a water slurry, or suspension, of cellulosic fibers (known as the paper “stock”) is fed onto the top of the upper run of an endless belt of woven wire and/or synthetic material that travels between two or more rolls. The belt, often referred to as a “forming fabric,” provides a papermaking surface on the upper surface of its upper run which operates as a filter to separate the cellulosic fibers of the paper stock from the aqueous medium, thereby forming a wet paper web. The aqueous medium drains through mesh openings of the forming fabric, known as drainage holes, by gravity or vacuum located on the lower surface of the upper run (i.e., the “machine side”) of the fabric.
After leaving the forming section, the paper web is transferred to a press section of the paper machine, where it passes through the nips of one or more pairs of pressure rollers covered with another fabric, typically referred to as a “press felt.” Pressure from the rollers removes additional moisture from the web; the moisture removal is often enhanced by the presence of a “batt” layer of the press felt. The paper is then transferred to a dryer section for further moisture removal. After drying, the paper is ready for secondary processing and packaging.
Typically, papermaker's fabrics are manufactured as endless belts by one of two basic weaving techniques. In the first of these techniques, fabrics are flat woven by a flat weaving process, with their ends being joined to form an endless belt by any one of a number of well-known joining methods, such as dismantling and reweaving the ends together (commonly known as splicing), or sewing on a pin-seamable flap or a special foldback on each end, then reweaving these into pin-seamable loops. A number of auto-joining machines are available, which for certain fabrics may be used to automate at least part of the joining process. In a flat woven papermaker's fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross machine direction.
In the second basic weaving technique, fabrics are woven directly in the form of a continuous belt with an endless weaving process. In the endless weaving process, the warp yarns extend in the cross machine direction and the filling yarns extend in the machine direction. Both weaving methods described hereinabove are well known in the art, and the term “endless belt” as used herein refers to belts made by either method.
Effective sheet and fiber support are important considerations in papermaking, especially for the forming section of the papermaking machine, where the wet web is initially formed. Additionally, the forming fabrics should exhibit good stability when they are run at high speeds on the papermaking machines, and preferably are highly permeable to reduce the amount of water retained in the web when it is transferred to the press section of the paper machine. In both tissue and fine paper applications (i.e., paper for use in quality printing, carbonizing, cigarettes, electrical condensers, and like) the papermaking surface comprises a very finely woven or fine wire mesh structure.
In a conventional tissue forming machine, the sheet is formed flat. At the press section, 100% of the sheet is pressed and compacted to reach the necessary dryness and the sheet is further dried on a Yankee and hood section. This, however, destroys the sheet quality. The sheet is then creped and wound-up, thereby producing a flat sheet.
In an ATMOS system, a sheet is formed on a structured or molding fabric and the sheet is further sandwiched between the structured or molding fabric and a dewatering fabric. The sheet is dewatered through the dewatering fabric and opposite the molding fabric. The dewatering takes place with air flow and mechanical pressure. The mechanical pressure is created by a permeable belt and the direction of air flow is from the permeable belt to the dewatering fabric. This can occur when the sandwich passes through an extended pressure nip formed by a vacuum roll and the permeable belt. The sheet is then transferred to a Yankee by a press nip. Only about 25% of the sheet is slightly pressed by the Yankee while approximately 75% of the sheet remains unpressed for quality. The sheet is dried by or on a Yankee/Hood dryer arrangement and then dry creped. In the ATMOS system, one and the same structured fabric is used to carry the sheet from the headbox to the Yankee dryer. Using the ATMOS system, the sheet reaches between about 35 to about 38% dryness after the ATMOS roll, which is almost the same dryness as a conventional press section. However, this advantageously occurs with almost 40 times lower nip pressure and without compacting and destroying sheet quality. Furthermore, a big advantage of the ATMOS system is that it utilizes a permeable belt which is highly tensioned, e.g., about 60 kN/m. This belt enhances the contact points and intimacy for maximum vacuum dewatering. Additionally, the belt nip is more than 20 times longer than a conventional press and utilizes air flow through the nip, which is not the case on a conventional press system.
Actual results from trials using an ATMOS system have found that the caliper and bulk of the sheet is 30% higher than the conventional through air drying (TAD) formed towel fabrics. Absorbency capacity is also 30% higher than with conventional TAD formed towel fabrics. The results were the same whether one uses 100% virgin pulp up to 100% recycled pulp. Sheets can be produced with basis weight ratios of between 14 to 40 g/m2. The ATMOS system also provides excellent sheet transfer to the Yankee working at 33 to 37% dryness. There is essentially no dryness loss with the ATMOS system since the fabric has square valleys and not square knuckles (peaks). As such, these is no loss of intimacy between the dewatering fabric, the sheet, the molding fabric, and the belt. A key aspect of the ATMOS system is that it forms the sheet on the molding fabric and the same molding fabric carries the sheet from the headbox to the Yankee dryer. This produces a sheet with a uniform and defined pore size for maximum absorbency capacity.
U.S. patent application Ser. No. 11/753,435 filed on May 24, 2007, the disclosure of which is hereby expressly incorporated by reference in its entirety, discloses a structured forming fabric for an ATMOS system. The fabric utilizes an at least three float warp and weft structure which, like the prior art fabrics, is symmetrical in form.
U.S. Pat. No. 5,429,686 to CHIU et al., the disclosure of which is hereby expressly incorporated by reference in its entirety, discloses structured forming fabrics which utilize a load-bearing layer and a sculptured layer. The fabrics utilize impression knuckles to imprint the sheet and increase its surface contour. This document, however, does not create pillows in the sheet as required for effective dewatering of TAD applications, nor does it teach using the disclosed fabrics on an ATMOS system and/or forming the pillows in the sheet while the sheet is relatively wet and utilizing a hi-tension press nip.
U.S. Pat. No. 6,237,644 to HAY et al., the disclosure of which is hereby expressly incorporated by reference in its entirety, discloses structured forming fabrics which utilize a lattice weave pattern of at least three yarns oriented in both warp and weft directions. The fabric essentially produces shallow craters in distinct patterns. This document, however, does not create deep pockets which have a three-dimensional pattern, nor does it teach using the disclosed fabrics on an ATMOS system and/or forming the pillows in the sheet while the sheet is relatively wet and utilizing a hi-tension press nip.
International Publication No. WO 2005/035867 to LAFOND et al., the disclosure of which is hereby expressly incorporated by reference in its entirety, discloses structured forming fabrics which utilize at least two different diameter yarns to impart bulk into a tissue sheet. This document, however, does not create deep pockets which have a three-dimensional pattern. Nor does it teach using the disclosed fabrics on an ATMOS system and/or forming the pillows in the sheet while the sheet is relatively wet and utilizing a hi-tension press nip.
U.S. Pat. No. 6,592,714 to LAMB, the disclosure of which is hereby expressly incorporated by reference in its entirety, discloses structured forming fabrics which utilize deep pockets and a measurement system. However, it is not apparent that the disclosed measurement system is replicatable. Furthermore, LAMB relies on the aspect ratio of the weave design to achieve the deep pockets. This document also does not teach using the disclosed fabrics on an ATMOS system and/or forming the pillows in the sheet while the sheet is relatively wet and utilizing a hi-tension press nip.
U.S. Pat. No. 6,649,026 to LAMB, the disclosure of which is hereby expressly incorporated by reference in its entirety, discloses structured forming fabrics which utilize pockets based on five-shaft designs and with a float of three yarns in both warp and weft (or variations thereof). The fabric is then sanded. However, LAMB does not teach an asymmetrical weave pattern. This document also does not teach using the disclosed fabrics on an ATMOS system and/or forming the pillows in the sheet while the sheet is relatively wet and utilizing a hi-tension press nip.
International Publication No. WO 2006/113818 to KROLL et al., the disclosure of which is hereby expressly incorporated by reference in its entirety, discloses structured forming fabrics which utilize a series of two alternating deep pockets for applications in TAD. However, KROLL does not teach to utilize one consistent sized pocket in order to provide effective and consistent dewatering and would not produce a regular sheet finish on the finished product. This document also does not teach using the disclosed fabrics on an ATMOS system and/or forming the pillows in the sheet while the sheet is relatively wet and utilizing a hi-tension press nip.
International Publication No. WO2005/075737 to HERMAN et al. and U.S. patent application Ser. No. 11/380,826 filed on Apr. 28, 2006, the disclosure of which are hereby expressly incorporated by reference in their entireties, disclose structured molding fabrics for an ATMOS system which can create a more three-dimensionally oriented sheet. These documents, however, do not teach, among other things, the deep pock weaves according to the invention.
International Publication. No. WO 2005/075732 to SCHERB et al., the disclosure of which is hereby expressly incorporated by reference in its entirety, discloses a belt press utilizing a permeable belt in a paper machine which manufactures tissue or toweling. According to this document, the web is dried in a more efficient manner than has been the case in prior art machines such as TAD machines. The formed web is passed through similarly open fabrics and hot air is blown from one side of the sheet through the web to the other side of the sheet. A dewatering fabric is also utilized. Such an arrangement places great demands on the forming fabric because the pressure applied belt press and hot air is blown through the web in the belt press. However, this document does not teach, among other things, the deep pock weaves according to the invention.
The above-noted conventional fabrics limit the amount of bulk that can be built into the sheet being formed due to the fact that they have shallow depth pockets compared to the instant invention. Furthermore, the pockets of the conventional fabrics are merely extensions of the contact areas on the warp and weft yarns.